In the past it has been known to provide threaded articles with a deposit of friction-producing resin material in the thread grooves of a threaded article to a depth which interferes with the crests of the threads of a mating article.
A commercial application of this practice has involved heating a threaded article to a temperature well above the softening or melting temperature of the resin, and applying a resin powder such for example as nylon to an area of the threaded surface. The nylon powder is fused or melted as it reaches the hot thread surfaces and builds up a deposit of substantially fluid melted resin. When the article is cooled or permitted to cool, the melted resin solidifies into a substantially continuous solid deposit primarily at the bottom of the thread grooves shaped and dimensioned to provide frictional interference with at least the crests of the thread of a mating member. Such an operation is disclosed in U.S. Pat. Nos. 3,093,177, 3,294,139, 3,416,492, 3,498,352, 3,554,258, 3,579,684 and 3,858,262.
The complications introduced in providing apparatus for the large scale production of threaded articles provided with a friction-producing deposit by requiring the application of the powdered resin to a heated article have left much to be desired.
In accordance with applicant's prior application Ser. No. 892,505, these complications are largely avoided by a method which comprises applying a relatively thick viscous mixture of a fluid, usually water, and resin particles to the area of a threaded article in which a friction producing deposit is applied while the threaded article is cool, thereafter drying the mixture to form a dry substantially solid deposit of discrete particles of resin powder, subsequently heating an article to fuse or melt the resin particles to form a fluid substantially continuous deposit of fused or melted resin, and finally cooling the article to produce a solid substantially continuous deposit of the friction-producing resin, concentrated primarily in the bottom of the thread grooves. Although not usually required, the resin powder may contain a binder which improves the resistance of the dried deposit to fracture or crumbling prior to the subsequent step of fusion.
In carrying out this procedure it was found that in some cases deposit of the fluid resin powder mixture by procedures compatible with high quantity production tended to produce uneven or irregular deposits. It appeared that the material deposited had a tendency to gather or ball-up, particularly when the surfaces of the threads were what is referred to in the trade as phosphate surfaces.
In industry and particularly in the automotive industry, large quantities of threaded articles are treated so that the thread surfaces thereof are rough phosphate coated surface type.
In order to overcome these difficulties and to permit application of the resin powder mixture to form uniform deposits, it has been found that surprisingly increased uniformity of the deposits is obtainable by pre-coating the thread surfaces to which the resin powder mixture is to be deposited with an alcohol solution of certain polymeric materials to provide a thin uniform film. These materials comprise the group consisting of acrylic material, polyesters, polyvinyl acetates and nylon. Of these the acrylic material has been found to be the most efficient by a considerable margin, although the other polymeric materials produced improved results.
The alcohol solution is quite dilute, containing from 0.5 to 10% of the polymeric material and the inclusion of 1-3% by volume has been found entirely suitable. The application of the alcohol solution produces an extremely thin uniform film over the thread surfaces.
Following the provision of the film as just directed, the essentially aqueous mixture of the powder resin is applied and will flow to produce a substantially uniform deposit concentrated in the bottom of the thread grooves shaped and dimensioned to provide the required interference with at least the crests of the threads of the mating threaded article.